Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density and high voltage have become commercialized and widely used. A lithium secondary battery generally uses a lithium transition metal oxide as a cathode active material and a graphite-based material as an anode active material.
LiCoO2 has been used as a typical cathode active material of the lithium secondary battery. However, LiCoO2 has many limitations in that it is relatively expensive, the amount of charge and discharge current is low at about 150 mAh/g, its crystal structure is unstable at a voltage of 4.3 V or more, and it has the risk of fire by causing a reaction with an electrolyte solution. Furthermore, LiCoO2 may exhibit very large changes in physical properties even if some parameters are changed during a manufacturing process. In particular, cycle characteristics at high potential and high-temperature storage characteristics tend to be significantly changed due to some changes in the process parameters.
In relation to the foregoing, a technique of coating an outer surface of LiCoO2 with a metal (aluminum etc.) in order to be operable at high potential, or a technique of heat treating LiCoO2 or mixing LiCoO2 with other materials has also been suggested. However, secondary batteries composed of such cathode materials may exhibit poor safety or may not be suitable for mass production.
In addition, transition metal compounds, such as LiNiO2, LiMn2O4, LiFePO4, LiNixCox-1O2 (where x=1, 2), and LiNi1-x-yCoxMnyO2 (where 0≦x≦0.5, 0≦y≦0.5), are used as a cathode active material for a lithium battery. Also, a composite-based oxide including an excessive amount of lithium has been suggested as an alternative as high capacity tends to be required.
However, with respect to the composite-based oxide including an excessive amount of lithium, since the electrical conductivity decreases due to local structural changes in the excessive amount of the cathode active material which occur in an activation operation, rate capability may decrease. Therefore, there is a need to address limitations in the conductivity in order to efficiently use the composite-based oxide as an active material.